CHAPTER

9

Informal Geometry

Geometric Figures

The word ‘‘geometry’’ is derived from two Greek words meaning ‘‘earth

measure.’’ The basic geometric figures are the point, the line, and the

plane. These figures are theoretical and cannot be formally defined. Apoint

is represented by a dot and is named by a capital letter. Alineis an infinite set

of points and is named by a small letter or by two points on the line. Aline

segmentis part of a line between two points called endpoints. Aplaneis a flat

surface (see Fig. 9-1).

Points and line segments are used to make geometric figures. The geo-

metric figures presented in this chapter are the triangle, the square, the rec-

tangle, the parallelogram, the trapezoid, and the circle (see Fig. 9-2).

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Fig. 9-1.

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